J Sainsbury, the supermarket chain, is in advanced talks to offload a portfolio of prime retail sites to a London-listed real estate investor for about £500m.

Sky News has learnt that LXi REIT, which has a market value of about £2.5bn, is close to agreeing a deal to acquire the freeholds to nearly 20 Sainsbury’s stores, which the grocer then intends to lease back.

A property industry source said on Tuesday evening that LXi was likely to seek to raise hundreds of millions of pounds in equity to fund the acquisition, with part of the consideration also including an unspecified amount of debt.

If confirmed, the deal would come months after it emerged that Sainsbury’s was exploring a sale of the portfolio.

A number of other parties are said to have expressed an interest in acquiring it.

Supermarket groups are under pressure from investors to improve the efficiency of their balance sheets, with both Asda and Wm Morrison having been acquired by new owners in the past two years.

LXi REIT has been active this year, agreeing a takeover of peer Secure Income REIT several months ago.

Among the prominent assets it owns are properties which form part of Merlin Entertainment, the group behind the Alton Towers, Legoland and Thorpe Park theme parks.

It invests in commercial real estate assets which benefit from long leases, typically lasting decades, and is a member of the FTSE-250 index.

LXI and Sainsbury’s both declined to comment.